Liquid crystal displays (LCDs) have found widespread usage in the prior art, one such display being, for example, of the twisted nematic liquid crystal type. Such LCDs operate by applying an alternating voltage potential between opposing electrodes sandwiching a liquid crystal layer therebetween. Twisted nematic LCDs used, for example, in cockpits of air vehicles typically include a matrix array of pixels and a corresponding backlight for illuminating the elements. These pixels are often temperature dependent with respect to their normal operating characteristics in that an LCD relies upon the behavioral characteristics of its twisted nematic layer as it is exposed to driving voltages. Because such voltage-related behavior of the liquid crystal (LC) is a function of temperature, the overall performance of the corresponding display is temperature dependent. When below a predetermined temperature, twisted nematic LC material does not behave in a consistent manner. Accordingly, the LC material in such situations must be heated to a predetermined temperature in order to achieve satisfactory functionality.
LCD heaters are known in the art. For example, see U.S. Pat. Nos. 4,643,525; 4,727,835; 5,247,374; and 5,523,873.
LCD heaters, including a conductive ITO coating deposited on a transparent glass substrate, are old and well-known in the art. In such heaters, the indium tin oxide (ITO) heater coating is typically energized by way of a pair of parallel bus bars aligned along a planar surface of the ITO. Such bus bars have typically been made of either silkscreened conductive epoxy or deposited metal, the deposited metal including, for example, a tri-layer combination of chrome, nickel, and gold. Other conventional heater bus bars have been composed of a silver frit (fired silver) deposited onto a piece of glass, with an ITO layer then deposited over the entire surface. Other bus bars have been fabricated using silver epoxy, thin film gold, thin film platinum, ultrasonic indium, and flexible circuitry.
Unfortunately, silver frit bus bars are costly and require special expensive indium solder to be used to attach wires to it. This special solder is expensive, and thus undesirable. Prior art heaters where the silver bus bars are deposited directly onto the glass are undesirable due to thermal stresses created in the silver-to-solder joint when the heater is turned on. It is these thermal stresses that create the need for the special indium solder used to connect the wires to the bus bars. It has been found that wires attached with normal conventional solder (e.g. SN63)often fall off during evaluation testing.
Accordingly, there exists a need in the art for an LCD heater including bus bars attached to a conductive film, wherein the presence or effect of thermal stresses are substantially reduced or even eliminated. Furthermore, there exists a need in the art for a cost effective heater which satisfies the above-listed needs, and may be implemented with conventional non-special solder, and overcomes step coverage reliability problems.